The prior art includes many different power amplifier designs, including balanced radio frequency (RF) power amplifiers. One well known balanced RF power amplifier design includes two individual amplifiers in parallel. The amplifiers are driven/combined with 90-degree hybrid circuits and/or couplers to form the balanced amplifier. Such balanced power amplifiers are commonly deployed in wireless handset transmitters (e.g., for cellular telephones, mobile computing devices, and the like).
Power amplifiers contained in mobile handset transmitters are exposed to loading effects arising from the antenna and external environment. The proximity of the antenna to various reflective surfaces results in a terminating impedance for the power amplifier that can vary significantly from the desired value (e.g., 50 ohms). The output power capability (to first order) of the amplifier depends on the terminating load impedance (or equivalently load line), and hence performance degradation occurs with antenna loading. Therefore, under certain antenna loading conditions, the power performance of the amplifier becomes compromised leading to a degradation in transmitted signal quality, and a reduction in transmitted power that leads to reduced link margin.
Minimizing impedance variations at the output of a power amplifier results in better overall linearity, which in turn results in increased link margins and, in a cellular telephone context, less dropped calls. Minimizing impedance variations also reduces the amount of interference experienced by other wireless devices. Moreover, minimizing impedance variations results in more efficient operation and reduced power consumption by the wireless device. The prior art includes several balanced power amplifier architectures that address (to some extent) the effects load impedance variations exhibit on the performance of the overall amplifier. Some of these existing solutions utilize tuning elements in a feedback path from the output of the balanced amplifier. Another existing solution employs fixed matching networks in the two amplifier branches.